Brooks, oxidation of the collar buttons, how I can solve?

if you don't like/ care to preserve the patina on the metal you can just Brasso it off into shining brass.
if you care to preserve most of the darkened brass, you can just use a little rust penetrating oil like WD40 or contact spray like Philips service spray things like that on a q tip and rub it a bit I guess. I used WD40 on my brass eyelets that grows green patina like that with q tips It reduce the green stuff but didn't polished the eyelets into shiny new brass.

I didn't spray WD40 to the jacket, I spray it a bit on its own cap and dip Qtip into it.

 

I have had similar green goop appear on the brass parts of my Himel Chevalier.
All i did was scrape it off gently with a wooden kebab skewer, the green actually flakes off pretty easily, the point of the skewer allows you to really get close to the edge of the brass.
I would be very VERY careful using Brasso or WD40 next to leather, that sounds like a horrible "wooops" waiting to happen.

 

Carefully remove verdigris with a sharp instrument, the kebob stick is a good idea. Will use that in the future. Maybe dab some brasso or vinegar on the snap.

You can then apply some wax, I use Renaissance, to keep it from coming back. You definitely want to remove it as it will build and pop the snaps off.

 

Literally scratched it off with a fingernail. Old toothbrush will do the job too.

 

Don't 'chemical' it - use a qtip, a nail brush and bicarbonate of soda with a bit of white vinegar to shift it. Usually, you can use dental floss to get underneath the eyelet/rivet. It doesn't harm the brass or the leather unless left for a very long time. Not a cause for over-worrying. Do not use anything like Brasso or an abrasive.

 

I revise my view , dont chemical it if possible, there is risk to stain the leather of course. Unless you confident with your preciseness.

 

Q-tips and patience do the job. Was able to completely remove all the verdigris from an old Langlitz once. Can't recall if I used a little water or not. Definitely a doable job!